Mitsubishi ZF and ZG military engines

[Elan Vital]

Hi everyone,

I have uncovered a few documents on the development of the Mtsubishi ZF and ZG series engines. The 10ZF was used on the Type 74 tank, the 6ZF on the Type 75 SPH and the 4ZF on Type 73 APC and derivatives. The 8ZG mentionned in the related document is a predecessor to the 10ZG, which probably got an extra 2 cylinders for better performance in the Type 90 tank.

They must be some of the most interesting unusual designs I have ever seen. It's quite remarkable how the Japanese managed to make 2-cycle engines viable in AFVs while Britain failed.

 

[Nicknick]

Interesting find. Have to say, the design isn’t that unusual, quite similar two stroke Diesels were very common in the US. The EMD and Detroit Diesels also used the same arrangement with intake ports at the bottom of the cylinders and four exhaust valves in the head. They also used (some variants) a combination of roots blower and turbo charger.

Putting four exhaust valves in a air cooled two stroke engine sounds a bit insane in respect to cooling issues, but their head design is more oil cooled than air cooled (also true for the Continental CD265 engine…).

This two-stroke approach with exhaust valves in the head is still the preferred solution for very large and slow running marine engines, but it died out every where else (at least to my knowledge).

For higher engine speeds, scavenging consumes a lot of power (here, 1 bar pressure difference!) and the fuel consumption is high. Modern piston design enabled the use of very short pistons for lighter engines in four stroke engines whereas this two-stroke design requires long piston sleeves. The cylinder spacing is higher for the two-stroke design because the 360 ° intake ports need to be supplied with air. The mechanical strength is weekend by the intake system and the cylinder pressure is lower than that of four stroke engines.

I don’t have the numbers for power to weight ratio of 40-year-old tank engines, but 3,44 kg/kw doesn’t look too well in my view (870 PS; 2200 kg), but surly the power to volume ratio is more important in that application.

Exhaust emissions and oil consumption were surly not extremely important for tank engines, but that became a nail in the coffin for the highly appreciated EMD Diesels.

Edit: When you take a look on Fig.1 in the third document, you will find that the older MTU MB873 engine is far superior in the extremely important power per m³ criteria and also slightly better in power to weight ratio. I’m sure, it was as well much better in bsfc terms, so the results for the 8ZG engine weren’t that successful. The mentioned British two stroke engines were opposed piston engines (at least, these are the only ones I know from GB) which offer an higher potential but also more development risk.